The present invention relates to a supercharged two-stroke or four-stroke internal combustion engine having one or more cylinders, and operating by admitting a carburated mixture or by admitting fresh air with the direct or indirect injection of fuel. The invention is just as applicable to petrol engines equipped with spark plugs as it is to diesel engines which use compression ignition.
Although the invention is described hereinafter with more particular reference to a single-cylinder engine in the case of a two-stroke engine, which is well suited to all applications of small industrial engines intended for motorized cultivation, garden tools, lawn mowers, cutters, scrub clearers or the like, the invention is not in any way restricted thereto and is also applicable to two-stroke or four-stroke multi-cylinder in-line or V engines.
A two-stroke single-cylinder engine which operates with natural aspiration into the cylinder of a carburated mixture which passes through the crankcase is already known. This engine has a pipe for admitting the air/fuel mixture and a pipe for exhausting the burnt gases, both of which pipes open in the form of ports toward the bottom of the cylinder, near bottom dead center (BDC). The carburated mixture from the carburetor is drawn into the crankcase through a valve, during the upstroke of the piston which causes a depression in the crankcase, and is then delivered to the cylinder, during the downstroke of the piston, causing a raised pressure in the crankcase. During the downstroke of the piston, the mixture inlet ports are open at practically the same time as the exhaust ports, which means that about 20% of the mixture is discharged directly to the exhaust, leading to a high fuel consumption and a great deal of atmospheric pollution. The main advantage of this engine is its low cost, but new antipollution standards will ultimately spell the end for this type of engine.
Another known engine is of the loop scavenging type, which operates with a positive-displacement compressor, for example of the Roots type, making it easier to introduce the carburated mixture into the cylinder and to generate low-pressure supercharging. This engine also has a mixture inlet pipe and an exhaust pipe, the pipes both opening via ports toward the bottom of the cylinder. In this engine, the carburated mixture is admitted into the cylinder from the compressor, with an orientation such that the mixture experiences a loop-like upward rotating movement after the manner of a xe2x80x9cloop-the-loopxe2x80x9d in the cylinder, while the burnt gases from the previous cycle are discharged to the exhaust ports. The particular arrangement of the inlet and exhaust ports makes it possible for part of the admitted mixture not to be exhausted directly, and this reduces both fuel consumption and environmental pollution.
Yet another known engine is of the uniflow type, which also operates using a positive-displacement compressor. This engine has an inlet pipe connected at its upstream end to the compressor and at its downstream end to an inlet ring which opens via a number of ports toward the bottom of the cylinder, with an orientation such that the mixture is introduced with a great deal of rotational movement. The burnt gases are discharged at the top of the cylinder through one or more exhaust valves. This type of engine allows control over the filling of the cylinder and the possible recirculation of burnt gases, so as to obtain combustion which causes less pollution. Furthermore, when this type of engine is operating on the diesel cycle, introducing the air near the bottom of the cylinder makes it possible to obtain a great deal of air rotation, which is needed for obtaining good efficiency. This engine makes it possible to consume even less fuel than the loop-scavenging engine, and also makes it possible to reduce polluting emissions.
However, these last two types of engine cost far more than engines with transfer via the crankcase, because they contain more parts, particularly the compressor, and furthermore, in the case of the uniflow engine, valve control means. Furthermore, compressors of the Roots type are of low efficiency; for example, a two-stroke single-cylinder engine with a one-liter cylinder capacity and a power of 55 kW will consume 17 kW for driving the compressor. What is more, a Roots compressor does not operate beyond a pressure higher than 1.2 bar.
Finally, engines with exhaust and inlet valves are known and these are able to obtain the lowest consumptions and the lowest emissions, but this type of engine is also the most expensive because both the exhaust valves and the inlet valves have to be controlled. The efficiency of this engine is better because the control of the opening and closing of the valves using parts external to the cylinder means that the entire piston stroke can be used whereas with the previous engines in which admission was via ports, part of the compression stroke and of the expansion stroke was lost.
The object of the invention is to provide a supercharged two-stroke or four-stroke internal combustion engine, for example of the loop scavenging, uniflow or valve type, or of the four-stroke valves type, which allows the efficiency to be improved and the emissions to be reduced.
To this end, the subject of the invention is a two-stroke or four-stroke internal combustion engine, operating by admitting a carburated mixture or by admitting fresh air with the direct or indirect injection of fuel, the engine having at least one engine cylinder, an engine piston which executes a reciprocating movement in said engine cylinder, said engine piston coupled by a connecting rod to the wrist pin of a crankshaft so as to drive said crankshaft in rotation, and at least one compressor having a compressor cylinder and a compressor piston engaged in said compressor cylinder so as to define at least one variable-volume compression chamber, wherein said compression chamber is connected to said engine cylinder by an inlet pipe in order to supercharge the engine cylinder with carburated mixture or with fresh air, said inlet pipe ending at an inlet member of the engine cylinder, wherein said engine comprises a coupling means for coupling said compressor piston to said crankshaft, said coupling means arranged to drive said compressor piston in a reciprocating movement in said compressor cylinder as said crankshaft rotates so that, at least at a predetermined operating speed, a supercharging pressure generated by said compressor piston in the compression chamber and propagated through said inlet pipe, reaches a maximum value in said engine cylinder at substantially the same time as the inlet member of said engine cylinder is shut off.
This feature makes it possible to obtain a supercharged engine in which combustion is more complete, thus increasing efficiency and reducing exhaust pollution. The choice of producing the maximum pressure in the combustion chamber of the engine cylinder at substantially the same time as the inlet member is shut off makes it possible, for the desired operating speed, to optimize the amount of fresh air or carbureted mixture introduced into the engine cylinder in each cycle, while at the same time controlling the richness of the mixture, thus increasing the torque and mechanical power. It should be noted that a phase shift between the top dead center of the compression piston and the top dead center of the engine piston is chosen so as to obtain a maximum pressure in the engine cylinder at the time that the inlet member is shut off so that the geometric value of this phase shift can vary to a large extent as a function of numerous constructional and operational parameters of the engine and of the compressor.
According to a particular embodiment of the invention, the coupling means comprises a cam follower member connected to said compressor piston to drive said compressor piston, said cam follower member being kept in contact with a cam profile carried by said crankshaft during at least part of a rotation cycle of said crankshaft, said cam profile being designed to drive said compressor piston via the cam follower member, with a reciprocating movement in said compressor cylinder as said crankshaft rotates.
As a preference, the crankshaft has a counterweight part which is off-centered away from said wrist pin to balance said crankshaft, part of said cam profile being carried by said counterweight part.
The counterweight is a part of the crankshaft which always has a great deal of asymmetry with respect to the axis of rotation of the crankshaft. As a result, producing a cam profile with the desired shape on the counterweight does not involve significant modification to the structure of the crankshaft, and this makes it possible to reduce the cost of obtaining the compressor.
Advantageously in this case, the cam follower member has the overall shape of a U with two branches and collaborates with said counterweight part of the crankshaft on each side of said wrist pin via respective ends of the two branches of said cam follower member.
In this case, the two branches are spaced sufficiently to allow the wrist pin to pass between them as the crankshaft rotates. This embodiment allows the cam follower member to be balanced and the fact that there are two regions of contact with the crankshaft reduces the wear on the regions concerned.
As a preference in this case, the compressor piston is connected to said cam follower member practically at the middle of a base of said cam follower member connecting the two branches, so that an axis of said compressor piston is practically coplanar with an axis of the engine piston. This arrangement makes it possible to reduce the bulk of the engine equipped with the compressor by putting the engine cylinder and the compressor cylinder in one and the same plane orthogonal to the axis of rotation of the crankshaft, and angularly offset from one another, for example perpendicular to one another.
Advantageously, a crankcase, in which said crankshaft is mounted so that it can rotate, carries means for guiding said cam follower member in translation in an axial direction of the compressor cylinder.
As a preference, the compressor piston comprises a flexible sealed diaphragm, a peripheral edging of which is fixed in a sealed manner to a side wall of the compressor cylinder and at least one rigid plate fixed against a central part of said diaphragm, said at least one rigid plate being connected to said cam follower member so as to be driven back and forth with respect to the compressor cylinder, an intermediate part of said diaphragm located between said central part and said peripheral edging being able to deform as said at least one rigid plate moves.
According to one particular feature of the invention, the cam follower member is arranged between said compressor piston and said crankshaft, an elastic return means being arranged to return said compressor piston and said cam follower member toward said crankshaft.
As a preference, said elastic return means is a compressible spring arranged in said compression chamber and bearing on said compressor piston, or arranged between said cam follower member and a crankcase of said engine.
Advantageously, an abutment member is borne by a crankcase of said engine to stop said cam follower member at an abutment position during another part of said rotation cycle of the crankshaft in which said cam follower member is no more in contact with said cam profile.
Advantageously, the cam profile has an angular region which, when it collaborates with said cam follower member, brings said compressor piston into a position corresponding to the production of a boost pressure spike in said compression chamber, the angle of a dihedron, the vertex of which is formed by the axis of rotation of the crankshaft and the two half-planes of which extend one toward said wrist pin and the other toward said angular region of the cam profile, being calculated as a function of said predetermined operating speed and of a length of said inlet pipe so as to allow said boost pressure spike propagating through said inlet pipe between said compression chamber and said engine cylinder to reach said engine cylinder at practically the same time as said inlet member is shut off.
In general, the position at which the pressure spike is produced in the compression chamber lies in the compression stroke of the compressor piston and precedes its top dead center by an amount which depends in particular on the valves installed on the outlet side of said compression chamber. The result of this is that the angle of the dihedron, which is chosen so as to obtain the pressure spike in the engine cylinder at the time that the inlet member is shut off, can adopt numerous geometric values depending on the desired optimum operating speed, on the configuration of the inlet pipe, on the nature of the valves, etc.
According to another feature of the invention, said inlet member comprises at least one port arranged in a lower part of said engine cylinder so as to be uncovered by said engine piston when said engine piston is in a range around its bottom dead center, and to be shut off by said engine piston during the remainder of the cycle of said engine piston.
Alternatively, said inlet member comprises a controlled intake valve arranged at the top of said engine cylinder.
Advantageously, the predetermined operating speed corresponds to obtaining a maximum torque or a maximum mechanical power on the output shaft of said engine.
According to another group of embodiments of the invention, said coupling means comprises an eccentric mounted on the shaft of said crankshaft and a link rod articulated to the eccentric and coupled to the compressor piston.
As a preference in that case, the angle of a dihedron, the vertex of which is formed by the axis of rotation of the crankshaft and the two half-planes of which extend one toward the eccentric and the other toward the wrist pin is designed as a function of a length of said inlet pipe so as to obtain a phase shift between the top dead center positions of the engine and compressor pistons associated with the respective engine and compressor cylinders that are connected together by said inlet pipe, wherein said phase shift ensures that a supercharging pressure spike propagating through said inlet pipe between said compressor cylinder and said engine cylinder reaches said engine cylinder at substantially the same time as said inlet member is shut off when the engine operates at said predetermined speed.
Advantageously, the cylinder capacity of the compressor is of the order of magnitude of that of the cylinder, but with a compressor piston which has a diameter markedly greater than the diameter of the engine piston, so that the compressor piston has a short compression stroke in the compression chamber.
In a particular embodiment, the compressor piston is rigidly attached at its center to the link rod for connection with the eccentric so that the compressor piston moves in the compression chamber by rocking back and forth about lower and upper parts of the compression chamber, the axis of the compressor being offset, in the direction of the axis of the crankshaft, with respect to the axis of the cylinder. In this case, the compressor piston can have, at its periphery, a spherical edging fitted with a spherical sealing ring which is preferably unable to rotate with respect to the compressor piston, in a position such that the gap in the ring is not placed at the bottom of the compressor, so as to limit the oil consumption and therefore the environmental pollution.
In another embodiment, the compressor piston is secured at its center to a rod articulated to the link rod for connection to the eccentric, said rod being guided in translation in a direction which intersects the axis of the cylinder. In a first alternative form, the compressor piston is a deformable diaphragm connected at its periphery to the side wall of the compression chamber, said diaphragm preferably having an undulation at its periphery, to make it easier to deform. In a second alternative form, the compressor piston is a rigid cylinder which can move in axial translation and is fitted at its periphery with at least one sealing ring.
This other embodiment is advantageous in that it carries no risk of oil passing between the crankcase and the compression chamber of the compressor, because it is possible to arrange a seal or a sealing boot on the compressor piston rod.
In one particular embodiment, the compression chamber has two stages located one on each side of the compressor piston, a first stage being supplied with carburated mixture or with fresh air by a first nonreturn valve or a valve, and connected by a delivery duct fitted with a second nonreturn valve or a valve to the second stage which communicates with the cylinder via an inlet duct possibly fitted with a third nonreturn valve or a valve. The use of a two-stage compressor makes it possible to obtain a higher boost pressure in the cylinder. However, in this case, the volumetric ratio of the cylinder may be reduced so as not to reach a maximum combustion pressure which is incompatible with the mechanical strength of the cylinder. The engine equipped with this two-stage compressor will work in a similar way to the known hyperbaric-type supercharging system.
The two-stroke engine of the invention may also be fitted with a device for recovering the energy in the exhaust puffs and for partially recirculating the exhaust gases by providing an additional volume communicating with the engine cylinder through closure and opening means, the movements of which are controlled either in synchronism or with a phase shift with respect to those of the engine piston in the engine cylinder so that during the expansion phase, the burnt gases compress the air in the additional volume and at least partially enter it, so that this air and burnt gases mixture is trapped under pressure therein, and then so that this mixture is admitted into the engine cylinder during the compression phase.
Advantageously, after the air and burnt gases mixture previously trapped in the additional volume has been admitted into the engine cylinder, said additional volume is once again filled with fresh air from the compressor.
According to another feature, the aforementioned closure and opening means comprise two rotary shutters, for example multi-way rotary spools, connected to each other by the additional volume, one of the shutters being associated with the compressor, and the other shutter being associated with the exhaust from the engine cylinder.
As a preference, the two rotary shutters are arranged in such a way that the following operations take place: in a first phase, when the engine piston is near its TDC, a flow of air from the compressor passes through the lower shutter associated with the compressor, sweeps through the additional volume, passes through the upper shutter associated with the exhaust and is exhausted to the outside via an exhaust manifold; in a second phase, from about halfway through the expansion stroke of the engine piston, on the one hand, the upper shutter places the engine cylinder in communication with the additional volume so as to fill it with a pressurized mixture of air and burnt gases and, on the other hand, the engine cylinder communicates with the exhaust; in a third phase, the upper shutter traps the air and burnt gases mixture in the additional volume; in a fourth phase, air from the compressor is admitted into the engine cylinder and, in a fifth phase, at the start of the engine piston compression stroke, the trapped and pressurized mixture is admitted into the engine cylinder.
In a first alternative form, the upper shutter is associated with at least one exhaust valve located at the top of the engine cylinder and the lower shutter is connected to the engine cylinder by a pipe arranged toward the bottom of the cylinder so that the additional volume is pressurized via its upper end by the burnt gases from the exhaust valve through the upper shutter and is emptied into the engine cylinder via its lower end through the lower shutter.
In a second alternative form, the upper shutter is connected to the engine cylinder by a pipe arranged toward the bottom of the cylinder and the lower shutter is fitted on the delivery pipe between the two stages of the compressor so that the additional volume is pressurized by means of the burnt gases from the engine cylinder through the upper shutter and is emptied into the cylinder through the pipe connected to the upper shutter.
Advantageously, in the case of two-stroke or four-stroke engines, the inlet pipe to the engine cylinder and/or the delivery pipe from the two-stage compressor is cooled by any appropriate means.
The two-stroke engine may be of the loop scavenging type, in which the carburated mixture or the fresh air is admitted from the compressor through an inlet duct opening via ports into the lower part of the engine cylinder with an orientation such that the mixture or the air is introduced with a looping upward rotating movement, while the burnt gases from the previous cycle are discharged through exhaust ports also arranged toward the bottom of the cylinder.
The two-stroke engine may alternatively be of the uniflow type, in which the carburated mixture or the air is admitted toward the bottom of the engine cylinder through inlet ports distributed at the base of the engine cylinder and supplied by a ring, itself connected to the compressor, while the burnt gases from the previous cycle are discharged through one or more exhaust valves located at the top of the cylinder.
Finally, the two-stroke or four-stroke engine may be of the type with exhaust and inlet valves, in which the valves are located at the top of the engine cylinder and the inlet valve or valves are supplied by the compressor.
The invention is also applicable to an engine of the type with several in-line engine cylinders, in which the compressors associated with each engine cylinder are arranged alternately on each face of the crankcase.